In apparatus with mutually moving parts which are connected to each other via a bearing and are subjected to different electrical potentials, there is the risk that a current path can occur through the bearing. The current passing the bearing surfaces may be of magnitude that the bearing is damaged by pitting of the bearing surfaces. It is known to protect the bearings by insulating the bearing seatings in the cases where the voltage difference is small, as is the case, for example, for asynchronous machines. This is not satisfactory when there are large voltage differences, since there is the risk of flashover. Examples of this are to be found in wind turbines and radar antennae which are subjected to lightning.